The present invention generally concerns improvements in scanning experiments, and more particularly, in using a rotating unbalanced mass for controllably generating predetermined scan patterns for experiments supported on balloons, free-flying spacecraft, or space platforms like a space shuttle or space station.
Various support platforms have been previously used for receiving experiments. Examples of such are balloons, freeflying spacecraft, or a space shuttle or space station. Typically, a given experiment might be supported in a gimbal mount on a balloon, or a space shuttle or space station. Such arrangement permits movement of the experiment relative to the support platform. In other instances, such as for example in connection with a free-flying spacecraft, the experiment may be more directly or fixedly supported on the spacecraft for movement therewith.
Frequently, it is desired to cause the line of sight of a given experiment to be scanned in a predetermined scan pattern, such as a circular or line pattern. Such scanning may be necessary to meet scientific objectives of the experiment. Furthermore, the particular experiment itself will often dictate the type or nature of scanning required.
In some instances, it is necessary to physically move or manipulate the entire support platform, such as a free-flying spacecraft, in order to obtain or achieve the required scan motion for the supported experiment. Such is typical for example of X-ray and gamma-ray experiments on spacecraft. In other instances, such as where a gimbal mount or equivalent arrangement is utilized, the position of the experiment must be manipulated relative to its support.
Three general types of scan patterns are well-known and commonly utilized in various circumstances. The first is known as a circular scan, which means that the line of sight of the experiment (or a spacecraft) is repeatedly traced out in a circle, preferably centered on a desired target. The radius and period of the scan are typically controlled characteristics.
A second type of scan pattern commonly known and used is called the line scan, in which the experiment line of sight repeatedly moves back and forth in a line centered on a target. The amplitude of the line sweep is normally controlled as a desired characteristic, as is the orientation of the sweep line and the scan period. Similarly, the radius of a circular scan is typically a controlled characteristic.
A third form of scan pattern is called the raster scan, which is based on a line scan further complemented with some relatively slower motion in a direction generally perpendicular to the original line scan. Such complementary motion may be generated through stepping, a constant velocity (relatively slow velocity) movement component, or a saw-toothed movement component.
Prior art methods for achieving such various scan motions often are variously limited, or have practical drawbacks in connection with factors such as power, weight, cost, performance, stability, or combinations of such factors. Typically, the same system used for scanning is also used for target acquisition and centering, which is often very inefficient. Well-known examples of such prior methods available such as for scanning balloon-borne experiments are control moment gyroscopes (CMG's), reaction wheels, torque motors, or a combination of such devices. One particular drawback of the CMGs is their relative expense. Reaction wheels are likewise a potentially expensive approach, and have very inefficient power consumption. In other words, reaction wheels can require a great deal of power, particularly depending on the physical dimensions of the experiment and the characteristics of the desired scan motion (for example, amplitude and period of the scan motion).
One of the simplest of the prior art approaches is to use torque motors for scanning the experiment by torquing it against a gondola, the structure between the experiment and the flight train which attaches to the balloon. However, an inherent limitation is involved that unless the inertia of the gondola is relatively much larger than that of the experiment, then the gondola may move as much (or more than) the experiment. Another somewhat related disadvantage is that the gondola can become involved in rocking motion and/or pendulous oscillations by virtue of the dynamics of the gondola and the flight train. Such disturbances can in turn be fed back into the experiment being scanned, causing obvious and undesired disadvantages in performance and/or stability.
In connection with scanning experiments supported on free-flying spacecraft, prior methods typically use CMG's, reaction wheels, or a reaction control system (RCS). The drawbacks associated with CMG's and reaction wheels used in connection with free-flying spacecraft are generally the same as those described above in connection with their use with balloons. The problems relating to use of an RCS is that it is usually quite expensive. Even more protracted of a problem since a free-flying spacecraft is involved, is that the RCS may require a large amount of propellant, and hence a large amount of weight and space (both constituting payload problems), in order to be able to scan for relatively longer periods of time. Also, the on-off characteristics of an RCS are such that it is an impractical device where relatively precise scanning is desired (or necessary).
Generally the same types of prior scanning methods available for scanning experiments gimbal mounted on a balloon have been used for scanning experiments gimbal mounted on a space platform like a space shuttle or a space station. Accordingly, the problems discussed above in connection with such circumstances generally persist in the space shuttle or space station environment, with the following exception. The torque motor approach in connection with a space shuttle/station has available the relatively larger mass of the space shuttle or space station against which to provide torquing. Also, there is no flight train as in connection with the balloon to further complicate overall dynamics of the system. At the same time, such relative advantages over the balloon environment in connection with torque motors may be completely offset by particular performance or stability problems, such as caused by local structure flexibility of the space shuttle or space station, or by the large-amplitude/high-frequency reaction torques generated with torque motors.